The Phytotoxins are host-specific because they are toxic only to the host that is, susceptible to the pathogen (which produces the toxin), and if they induce nearly all symptoms of the disease then they are considered to be definitive chemical probes in a study of disease susceptibility and physiological stress at the molecular level (Bottinii A. T., and Gilchrist, D. G. 1981. Tetrahedron lett. 22:2719-2722). Physiological, biochemical, genetic and histological data all confirm that these toxic compounds produced by the fungi are the key determinants of disease and host selection (Scheffer, R. P., Kohmoto, K. and Durbin, R. D., 1989. ed. Host-Specific Toxins, pp.1-17).
The tolerance and sensitivity to a toxin is controlled by the same genes in the same way as they control resistance and susceptibility to the fungus (Scheffer, Robert P. and Livingston, Robert S.,1984. Science, 223:17-21). If a fungus does not grow on a plant, the phytotoxin produced by the fungus will not affect that plant, depending on whether or not it has dominant or recessive alleles (Grogan, R. G., Kimble, K. A., and Misaghi, I., 1975. Phytopathology, 65:880-886).
Members of the genus Lasiodiplodia are known to produce a wide range of phytotoxic compounds which affect a large number of plants on which the fungus is found (Domsch, K. H., and Ganms, W., 1980. Compendium of soil fungi, vol.1, Academic press, pp:143-145 ). Lasiodiplodia theobromae is a widespread soil-borne saprophyte or wound parasite in the tropics of all continents but has not been reported from the temperate zones of Europe and the U.S.A. It is common on fruits, stems and roots of numerous tropical and subtropical plants, particularly Theobromae, Castilla, Hevea, Citrus, Mangifera, Ficus and Musa (Goss, R. D., Cox, E. A. and Strotzky, 1961. Mycologia, 53:262-277; Laskin, A. I. and Lechevalier, H. A. (EDS) 1973, Handbook of Microbiology, Vol. 3, Microbial Products, CRC Press, Cleveland)
There are many fungi which are pathogenic to weeds because they produce phytotoxins and, therefore, could be used as herbicides (Abbas, H. K., Boyette, C. D., Hoagland, R. E., and Vesonder, R. F., 1991. Weed Sci. 39:673-677. The genus Aternaria is also known to produce a wide range of phytotoxins (Bruce, V. R., Stack, M. E., and Mislivec, P. B., 1984. J. Food Sci. 49:1626-1627. An isolate of Fusarium moniliforme, obtained from infected jimsonweed, was found to produce fumonisin phytotoxin.
The applicants in the course of their studies on Coleus forskohlii Briq. (Labiate) found that this plant is susceptible to attack by micro-organisms and most of the pathogens attacking the plant have not been identified. The applicants have also found that a fungus Lasiodiplodia theobromae which is responsible for the rot disease in coleus plants, releases certain toxins which exhibit phytotoxicity against weeds like Parthenium.